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Water, the most economical and effective extinguisher, on some fires, is inadequate or even harmful, as in the case of flammable liquids. For this reason foam extinguishing systems have been developed. An advantage of the foam is that in it a person can breathe; the air contained in the bubbles has the same mixture as the free one. Otherwise he can't do the fire. The wetting and cooling capacity of the foam is the main feature that determines the extinguishing effect; increasing the wetting effectiveness of the water, less can be used and greater resistance to the re-ignition of the flames.

In terms of removing heat from the fire, the efficiency of a foam system depends on the quality of the air bubbles; the smaller they are, regular, compact and numerous, the larger the heat exchange surface, the higher the extinguishing efficiency. In contact with the flames, the water contained in the bubbles converts into steam, increasing its volume by about 1700 times. Occupying the space, the vapor expels the air and lowers the concentration of oxygen to less than 7.5%, abundantly below the threshold necessary for the fire to sustain itself. Simultaneously with the effect of "suffocating" the water, vaporising absorbs heat, cooling the materials involved in the fire.

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How a foam extinguishing system works

It consists of foam generators, lances or nozzles. Other system architectures are the so-called water-foam sprinkler systems; similar to traditional sprinkler systems, they are supplied through a mixing device.

Low, medium and high expansion foams

Class A foams were developed in the mid-1980s to combat forest fires, to reduce surface water tension and to improve wetting capacity. However, they are not able to contain the explosive vapours produced by flammable liquids.

To obtain this result the surfactants, which allow the foam to form, must create a protective film on the objects and liquids on which it is poured (at the expense of lower flowability and speed of expansion). In low-expansion systems the bubble expansion ratio is lower (less than 20 to 1) and the bubble contains a high water content; in those with medium and high expansion, the ratios are higher (up to 1000 to 1) and the water content is lowered and the bubbles are relatively lighter.

Low-expansion foams have a low expansion rate, low viscosity and can quickly cover large areas; those with high expansion are suitable for closed spaces, where rapid filling is required.

A foam for each fire

Protein and fluoroprotein foaming agents experienced their greatest development and diffusion in the 1970s and 1980s, especially in the oil sector due to their use on large flammable liquid storage tanks. In the civil environment, in the engine rooms of ships or airport hangars, synthetic or "fluorosynthetic" surfactants are preferred - but biodegradable and non-toxic - which are very efficient; with less than 1% in concentration, particularly effective foams are obtained, despite the fact that the foam produced is very “liquid” and fluid, with a great resistance to re-ignition, thanks to the formation of an aqueous or polymeric film, which isolates the surface of the burning fuel from oxygen, also hindering the formation of ignitable vapours.

Extinguishing characteristics of a foam

What characterises a foam is its fire resistance, or the ability to keep the emulsion water incorporated. Its specific weight is another important variable that contributes to the spreading speed of the blanket and to the useful range for the same dispenser.

Spreading, or rather the ability to spread, and adhesion even on inclined planes, determine how the foam is able to cling, while the elasticity and impermeability to gases and vapours measure the ability of the blanket to resist the disintegrating action produced by the convective motions of the fire.

These characteristics can be affected by the contamination of foams by other substances, therefore research aims to develop foams that are increasingly compatible with other products (hydrocarbons involved in fire, other extinguishers such as dusts) and, not least, with the environment.

Furthermore, the viscosity of a foam is a fundamental requirement as it is directly proportional to its aspirability, especially at low temperatures.

The contribution to extinguishing flames is made, by the foam, by "deflating"; which it tends to do anyway, of course. Resistance to this process is one of the factors that contributes to the extinguishing capacity of the foam and is measured by a variable known as drainage time; the lower it is, or the earlier the air separates from the solution that traps it, the more the foam loses its suffocation characteristic.

Compressed air foam systems

As the foam is composed of water, a surfactant and air, enclosed in the bubbles, if in addition to foaming, compressed air is added to the water, further advantages in terms of effectiveness are obtained. In fact, this consists of the Compressed Air Foam System or CAFS systems. These systems are based on the control - guaranteed by electronic pump control systems - of the mixture. The standards classify CAFS systems essentially on the basis of the air flow rate of the compressor, with a water/air ratio not lower than 1:3.

The long history of fire foams

Oil was in the air, in the nostrils, in the eyes, in the water of the bath - everywhere, wrote Abraham Valentine Williams Jackson in 1911 about Baku, today's capital of Azerbaijan and then, for sixty years the centre the Russian oil industry and beyond; it was in fact from there that a fifth of the oil at the time used in the world came. With all that oil it is no wonder that fires were a difficult problem to solve but it was less than 10 years before the description given to us by Jackson, from Baku, that a fifty-five-year-old engineer graduate from the St. Petersburg Polytechnic, who then went on to study chemistry in Paris, developed an effective approach to combat them. In 1904 Aleksandr Grigoryevich Loran, who was a teacher in Baku, developed the extinguishing foam and built the first extinguisher based on this extinguishing agent; his first experiment was the successful extinguishing of a tank of naphtha, a mixture of hydrocarbons.

The Loran foam was a mixture of sodium bicarbonate and aluminium sulphate, with small amounts of saponin or licorice added to stabilise the bubbles, and of course, water. The reacting substances developed a stable solution of small bubbles of carbon dioxide with a density lower than that of oil or water. Lighter than oil, this foam flowed freely on the surface of the liquid, suffocating the fire. Loran patented his invention and subsequently founded a company - Eurica - in St. Petersburg, and began producing and selling his foam extinguishers.

At the beginning of World War II, foam fire extinguishers underwent radical innovation; the foam was produced by spraying through a nozzle a concentrate based on ​liquid soya proteins mixed with water. The inventor of this solution, Percy Lavon Julian, was a talented chemist and pioneer in the chemical synthesis of plant drugs (his work has helped to significantly expand the use of several important drugs and earned him over 130 patents) but decisive for the development of fire-fighting foam was the fact that he was an African American born and raised in segregationist America.

In 1936 Julian was denied a professorship for racial reasons and whilst seeking an academic position he was contacted by WJ O'Brien, vice president of Glidden Company, a supplier of soya-derived products. Julian had requested a sample of five gallons of oil to be used as a starting point for the synthesis of human steroid sex hormones. O'Brien offered him the position of director of research at Glidden's Soya Products Division in Chicago where Julian designed and supervised the construction of the first system in the world for the production of ​​soya isolate protein of industrial quality from soya flour without oil. A sample of soya protein isolate from Julian was sent by Glidden to the National Foam System Inc., that used it to develop Aer-O-Foam, the first protein-based fire foam. For the high-expansion foam, it was necessary to wait for another decade and another multi-faceted figure: Herbert Eisner.

Writer, television and radio scriptwriter, descendent of a family of Berlin entrepreneurs and intellectuals with acquaintances of the calibre of Richard Strauss, Bertholt Brecht and Leni Riefenstahl, he left Hitler's Germany, still a boy, in opposition to the Nazi regime and following racial persecution. Having graduated in physics from Nottingham University, Eisner joined the Safety in Mines Research Establishment in Buxton, Derbyshire, of which he became director. In his Health and Safety Executive's flame and explosion laboratory, he accumulated considerable experience in fires and explosions in confined spaces, which was fundamental to him in the development of high-expansion foam in 1956. In fact, in the United States, Will B. Jamison, a mining engineer from Pennsylvania, starting from the results of Eisner, tested 400 different surfactants for two years, with the collaboration of the .S. Bureau of Mines, until finding a suitable and patentable compound.

During the research two major mining fires were fought with remarkable success using the systems he was testing. Jamison devoted six more years to research before handing over the patent in 1964 to Walter Kidde & Company and spearheading the development, within the same company, of high-expansion foam systems. They were years of remarkable developments in the field of surfactants; while Jamison and Kidde continued on their path, National Foam, Inc. developed the fluoroprotein foam, a fluorinated surfactant that repels oil and prevents foam contamination (one of the problems faced by foam extinguishers) and ten years later it presented a synthetic foam developed for hydrocarbon and polar solvent materials (which are liquids that extract the water contained in the foam, breaking it up). A fundamental ingredient of foam is air; it was thought then, at a certain point, to insufflate it inside the fluids to enhance the generation of foams.

The first studies of a system that used water, compressed air and surfactants to produce foams date back to the 1930s in the maritime sector; The first testimony is documented in a 1938 article in the British magazine "The Fireman" on the "Pneumasuds" a type of fire-fighting equipment produced by Merrywheather and installed on board the luxurious passenger ship Patricia. The system was equipped with an electric motor, a double-acting piston pump for water, a rotary air compressor, a small foam pump, a Venturi device and a tank for the foam solution. The CAFS technology underwent further developments during the Second World War both in the navy and in the aeronautics sector. In Europe, in the 1940s, German companies such as Fladerer and Magirus were exploring the area of foam systems and the defeat of the III Reich caused an interruption of these studies while the projects were requisitioned by the winning powers.

This was not the only reason that led to a stall in the development of CAFS systems after World War II; only the advent of electronics made it possible to overcome the limits of the systems of synchronisation and fine control of the foam characteristics. In the 1970s, CAFS systems were installed on fire-fighting vehicles, especially in the United States and particularly in arid areas, where water-saving was essential. The ability of CAFS systems to generate “dry” foams also makes it possible to operate on difficult fires thanks to the ability of the foam to also “cling” to vertical slopes and to withstand for a long time thanks to its slow drainage. More dense foams were found to be better for protection while very fluid ones were ideal during extinguishing.

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